Polyvinyl acetal represented by polyvinyl butyral has excellent adhesiveness and compatibility with various organic and inorganic substrates and excellent solubility in an organic solvent. As such, it is widely used as various adhesives, and a binder for ceramics, various ink, paint, or the like, or an intermediate film of a laminated glass.
Recently, for giving various functions to an intermediate film of a laminated glass, a laminated intermediate film in which a plasticized polyvinyl acetal sheet and a sheet consisting of a resin other than polyvinyl acetals is laminated is being studied. For example, for the purpose of giving a high sound insulation property to an intermediate film of a laminated glass, studies are made regarding an intermediate film of a sound proof glass in which a plasticized polyvinyl acetal sheet and a styrene-diene block copolymer sheet having an excellent sound insulation property are laminated (refer to Patent Documents 1 to 3) or a highly sound proof front board of a pachinko machine in which such laminated glass is used (refer to Patent Document 4) is studied.
In the prior art literatures above, a plasticizer used for plasticized polyvinyl acetal is not specifically mentioned. However, when a carboxylic acid ester compound of monovalent alcohol and polyvalent carboxylic acid like dihexyl adipate and dioctyl phthalate that is commonly used as a polyvinyl acetal intermediate film of a laminated glass or a carboxylicacid ester compound of monovalent carboxylic acid and polyhydric alcohol like triethylene glycol di 2-ethylhexanoate (3G8) and triethylene glycol diheptanonate is used, the plasticizer migrates from a plasticized polyvinyl acetal sheet to a styrene-diene block copolymer sheet to cause a decrease in dynamic strength or an increase in haze, as shown in the Comparative Examples described below (Comparative Examples 1 to 8).
Further, as another intermediate film of a high performance laminated glass, studies are made on an intermediate film having improved penetration resistance by laminating a plasticized polyvinyl acetal sheet with polycarbonate or poly(meth)acrylic acid ester.
However, as shown in the Comparative Examples described below (Comparative Examples 1 to 8) and also pointed out in Patent Document 5, migration of a plasticizer occurs when a common plasticizer like those described above is used even for a laminate consisting of a plasticized polyvinyl acetal sheet and polycarbonate or poly(meth)acrylic acid ester, yielding problems like a decrease in dynamic strength of polycarbonate or poly(meth)acrylic acid ester or an increase in haze.
Under the purpose of inhibiting migration of a plasticizer from a resin composition added with a plasticizer to another resin, a plasticizer with relatively high molecular weight, i.e. a polymer plasticizer, may be used. As a composition including a plasticizer with high molecular weight, an intermediate film of a laminated film consisting of polyvinyl acetal and poly(ε-caprolactone) (trade name: PLACCEL H5, manufactured by Daicel Chemical Industries, Ltd.), that is an intermediate film of a laminated glass using a composition which consists of polycaprolactone having average polymerization degree of 10 to 2,000 as a polymer of polyvinyl butyral and ε-caprolactone, is disclosed (for example, refer to Patent Document 6). However, since the polycaprolactone used therein is highly crystalline and has relatively high melting temperature (melting point: 60° C.), the polycaprolactone contained in the intermediate film is crystallized to cause phase separation or the intermediate film has white cloudiness when the composition is kept at room temperature for a long period of time. Further, since the polycaprolactone has high melting temperature and also high polymerization degree, the effect of plasticizing polyvinyl acetal is not sufficient, and therefore the intermediate film of a laminated glass has only insufficient flexibility.
A ceramic binder using a composition consisting of polyvinyl butyral and polycaprolactone is disclosed (for example, refer to Patent Document 7) and a ceramic binder consisting of polyvinyl butyral and carboxylic acid polyester is disclosed (for example, refer to Patent Document 8). However, it is not expected by the invention disclosed in Patent Document 7 to obtain the effect of plasticizing the polyvinyl acetal by polycaprolactone. It describes only the use of poly(ε-caprolactone) having polymerization degree of 60 to 800. In fact, since the polycaprolactone used in the invention is highly crystalline and has relatively high melting temperature (melting point: 60° C.), it has an insufficient plasticizing effect for polyvinyl acetal and the polycaprolactone contained in the intermediate film is crystallized to cause phase separation when the composition is kept at room temperature for a long period of time. Further, it is not expected in Patent Document 8 that the ceramic binder thereof has an effect of plasticizing the polyvinyl acetal by polyester and the polyester described in the document is not shown to have an effect of plasticizing the polyvinyl acetal.
Further, a mixture of polyvinyl acetal and carboxylic acid polyester having no polar group like a hydroxyl group and a carboxyl group at the molecular end (i.e. carboxylic acid polyester having completely blocked terminal), which can be obtained by a reaction between dicarboxylic acid, a monovalent carboxylic acid, and a compound selected from 1,2-propane diol, 1,3-butane diol, and 1,4-butane diol, is disclosed (For example, refer to Patent Document 9). However, as shown in the Comparative Examples described below (Comparative Examples 2, 3, 9 and 10), when the average molecular weight of carboxylic acid polyester having completely blocked terminal is high, compatibility with the polyvinyl acetal is poor and phase separation may occur over time. There is also a problem in that the plasticizing effect is not sufficient. On the other hand, when the average molecular weight of carboxylic acid polyester having completely blocked terminal is low, there is a problem in that the migration resistance of a component having low molecular weight that is contained in the corresponding carboxylic acid polyester having completely blocked terminal is insufficient.